Variable resistance memory device having reduced bottom contact area and method of forming the same

ABSTRACT

A variable resistance memory element and method of forming the same. The memory element includes a substrate supporting a bottom electrode having a small bottom contact area. A variable resistance material is formed over the bottom electrodes such that the variable resistance material has a surface that is in electrical communication with the bottom electrode and a top electrode is formed over the variable resistance material. The small bottom electrode contact area reduces the reset current requirement which in turn reduces the write transistor size for each bit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.13/166,362, filed on Jun. 22, 2011, now U.S. Pat. No. 8,354,661 which isa divisional of U.S. patent application Ser. No. 11/798,459, filed onMay 14, 2007 (now U.S. Pat. No. 7,989,251), each of which isincorporated by reference herein in its entirety.

FIELD OF THE INVENTION

Embodiments of the invention relate to semiconductor devices, and inparticular to variable resistance memory elements and methods of formingand using the same.

BACKGROUND OF THE INVENTION

Non-volatile memories are important elements of integrated circuits dueto their ability to maintain data absent a power supply. Severaldifferent typical resistance memory elements using different materialshave been suggested for such use. Variable resistance memory elementscan be programmed to a resistance value as a way of storing logic data.One suggested type of resistance memory employs phase change materialssuch as chalcogenide alloys, which are capable of stably transitioningbetween amorphous and crystalline phases. Each phase exhibits aparticular resistance state and the resistance states distinguish thelogic values of the memory element. Specifically, an amorphous stateexhibits a relatively high resistance, and a crystalline state exhibitsa relatively low resistance.

FIG. 1A shows a basic composition of a variable resistance memory cell10, which may be implemented using a variable resistance material, andwhich is constructed over a substrate 11. A variable resistance material16 is formed between a bottom electrode 14 and a top electrode 18. Thebottom electrode is located within an insulating layer 12. The materialused to form the electrodes 14, 18 can be selected from a variety ofconductive materials, such as tungsten, nickel, tantalum, titanium,titanium nitride, aluminum, platinum, or silver, among others.

Much research has focused on variable resistance memory devices usingchalcogenides. Chalcogenides are alloys of Group VI elements of theperiodic table, such as tellunium (“Te”) or selenium (“Se”) or germanium(“Ge”). A specific chalcogenide currently used in rewriteable compactdiscs (“CD-RWs”) is Ge₂Sb₂Te₅. In addition to having valuable opticalproperties that are utilized in CD-RW discs, Ge₂Sb₂Te₅ also hasdesirable physical properties as a variable resistance material, i.e.,phase change material or GST material. For example, various combinationsof Ge, antimony (“Sb”) and Te may be used as variable resistancematerials. Specifically, GST material can change structural phasesbetween an amorphous phase and two crystalline phases. The resistance ofthe amorphous phase (“a-GST”) and the resistances of the cubic andhexagonal crystalline phases (“c-GST” and “h-GST,” respectively) candiffer significantly. The resistance of amorphous GST is greater thanthe resistances of either cubic GST or hexagonal GST, whose resistancesare similar to each other. Thus, in comparing the resistances of thevarious phases of GST, GST may be considered a two-state material(amorphous GST and crystalline GST), with each state having a differentresistance that can be equated with a corresponding binary state. Avariable resistance material such as GST whose resistance changesaccording to its material phase is referred to as a phase changematerial. The transition from one GST phase to another occurs inresponse to temperature changes of the GST material.

In a variable resistance memory cell, heating and cooling of the GSTmaterial can occur by causing differing amplitudes of current to flowthrough the GST material. The GST material is placed in a crystallinestate by passing a crystallizing current through the GST material, thuswarming the GST material to a temperature wherein a crystallinestructure may grow. A stronger melting current is used to melt the GSTmaterial for subsequent cooling to an amorphous state. As the typicalphase change memory cell uses the crystalline state to represent onelogic value, e.g., a binary “1,” and the amorphous state to representanother logic value, e.g., a binary “0,” the crystallizing current istypically referred to as a write current I_(W) and the melting currentis referred to as an erase or reset current I_(RST). One skilled in theart will understand, however, that the assignment of GST states tobinary values may be switched if desired.

The state of the GST material is determined by applying a small readvoltage V_(r) across the two electrodes and by measuring the resultantread current I_(r). A lower read current I_(r) corresponds to a higherresistance I_(r) where the GST material is in an amorphous state and arelatively high read current I_(r) corresponds to a lower resistancewhere the GST material is in a crystalline state.

The phase-changing current is applied to the GST material via a pair ofelectrodes. In FIG. 1B, for example, the phase-changing currents areapplied via the bottom electrode 14 and the top electrode 18. Because ofthe configurations of the bounding surface areas of the two electrodes14, 18, current densities 50 within the GST material are not equallydistributed. In particular, current densities 50 near the bottomelectrode 14 are greater than the densities near the top electrode 18.Furthermore, areas of the GST material that are directly in between thetwo electrodes 14, 18 have higher current densities 50 than areas thatare not directly in between the two electrodes 14, 18 such as areas nearthe lower corners of the GST material.

A sought after characteristic of non-volatile memory is low powerconsumption. Often, however, conventional variable resistance memoryelements require large operating currents. It is therefore desirable toprovide variable resistance memory elements with reduced currentrequirements. For variable resistance memory elements, it is necessaryto have a current density that will heat the variable resistancematerial past its melting point and quench it in an amorphous state. Oneway to increase current density is to decrease the size of the bottomelectrode. These methods maximize the current density at the bottomelectrode interface to the variable resistance material. Althoughconventional solutions are typically successful, it is desirable tofurther reduce the overall current in the variable resistance memoryelement, thereby reducing power consumption in certain applications.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B illustrate a prior art variable resistance memory cell.

FIG. 2 illustrates an embodiment of the invention described herein.

FIG. 3 illustrates the embodiment of FIG. 2 at an initial stage ofprocessing.

FIG. 4 illustrates the embodiment of FIG. 2 at a stage of processingsubsequent to that shown in FIG. 3.

FIG. 5 illustrates the embodiment of FIG. 2 at a stage of processingsubsequent to that shown in FIG. 4.

FIG. 6 illustrates the embodiment of FIG. 2 at a stage of processingsubsequent to that shown in FIG. 5.

FIG. 7 illustrates the embodiment of FIG. 2 at a stage of processingsubsequent to that shown in FIG. 6.

FIG. 8 illustrates the embodiment of FIG. 2 at a stage of processingsubsequent to that shown in FIG. 7.

FIG. 9 illustrates the embodiment of FIG. 2 at a stage of processingsubsequent to that shown in FIG. 8.

FIG. 10 illustrates the embodiment of FIG. 2 at a stage of processingsubsequent to that shown in FIG. 9.

FIG. 11 illustrates the embodiment of FIG. 2 at a stage of processingsubsequent to that shown in FIG. 10.

FIG. 12 illustrates the embodiment of FIG. 2 at a stage of processingsubsequent to that shown in FIG. 11.

FIG. 13A illustrates a top-down perspective view at the processing stateillustrated in FIG. 12 and FIG. 13B illustrates a top-down view at astage of processing subsequent to that shown in FIG. 13A, of a memoryarray according to an embodiment described herein.

FIG. 14 illustrates an electrical schematic of a memory cell accordingto an embodiment described herein.

FIG. 15 illustrates an array of phase change memory bit structuresaccording to an embodiment described herein.

FIG. 16 illustrates a processor system that includes a memory deviceaccording to an embodiment described herein.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, reference is made to variousspecific embodiments. These embodiments are described with sufficientdetail to enable those skilled in the art to practice the claimedinvention. It is to be understood that other embodiments may beemployed, and that various structural, logical and electrical changesmay be made.

The term “substrate” used in the following description may include anysupporting structure including, but not limited to, a semiconductorsubstrate that has an exposed substrate surface. A semiconductorsubstrate should be understood to include silicon, silicon-on-insulator(SOI), silicon-on-sapphire (SOS), doped and undoped semiconductors,epitaxial layers of silicon supported by a base semiconductorfoundation, and other semiconductor structures, including those made ofsemiconductors other than silicon. When reference is made to asemiconductor substrate or wafer in the following description, previousprocess steps may have been utilized to form regions or junctions in orover the base semiconductor or foundation. The substrate also need notbe semiconductor-based, but may be any support structure suitable forsupporting an integrated circuit structure, including, but not limitedto, metals, alloys, glasses, polymers, ceramics, and any othersupportive materials as is known in the art.

The embodiments of the invention are now explained with reference to thefigures, which throughout which like reference numbers indicate likefeatures. FIGS. 2-14 illustrate an example process for forming avariable resistance memory element 100 which has smaller electrodes thanother phase change memory devices and which can operate with resetcurrents which are smaller compared with other phase change memorydevices.

In order to reduce the reset current requirement for a writing bit, theembodiments herein describe process flows for producing a sub 1000 nm²bottom contact area that provides excellent contact area uniformityacross a wafer. The small bottom electrode contact area reduces thereset current requirement, which in turn reduces the write transistorsize for each bit. The resulting contact area is very scalable and theonly variation is the critical dimension (“CD”) control of the bit linesin one direction. Several methods of contact area reduction are known inthe art including contact hole CD reduction using a spacer, annulardonut shaped contacts, and edge contacts. The embodiments describedherein, however, provide advantages over the known methods and resultsin CD/area uniformity control and printing and etching only straightlines with only one direction CD control.

The resulting variable resistance memory element, e.g., phase changememory element, shown in an embodiment depicted by FIG. 2, includes asubstrate supporting bottom electrodes 145 having a sub 1000 nm² bottomcontact area. A phase change material 121 is formed over the bottomelectrodes 145 such that the phase change material 121 has a surfacethat is in electrical communication with the bottom electrode 145. A topelectrode 117 is formed over the phase change material 121 and is inelectrical communication with phase change material 121 and bottomelectrode 145. One method of forming the FIG. 2 structure is nowdescribed.

Referring to FIG. 3, a gate oxide material 130 is formed on top of asubstrate 110 having STI regions 120 formed therein. The gate oxidematerial 130 is removed from above the STI regions 120. Next, as shownin FIG. 4, a conductive material 140, e.g., a polysilicon material, isblanket deposited, and then a metal material 150, i.e., a Ti/WNmaterial, is blanket deposited. Then, a tungsten (W) material 160 isblanket deposited, and finally a nitride material 170 is blanketdeposited. Each material is blanket deposited above the previouslyformed material. Blanket deposition can be done by known methods.Materials 130, 140, 150, 160 and 170 comprise the materials that willform transistor gate stacks (described below). It should be appreciatedthat the gate stacks can also be formed in a number of differentcombinations and/or of different materials.

As shown in FIG. 5, nitride material 170 is etched except for areasabove where the desired gate stacks will be formed. Any etching methodknown in the art can be used such as using photolithography mask and wetor dry etching techniques. Using nitride material 170 as a mask,materials 140, 150 and 160 are etched by any technique, as shown in FIG.6, to form the desired gate stacks 171. Then, source/drain regions 180are formed in the substrate 110 on both sides of the gate stacks 171. Itshould be appreciated that the source/drain regions 180 are preferablyformed after gate stacks 171 have been formed, however, they may be alsoformed before the gate stack materials 130, 140, 150, and 160 areblanket deposited. A nitride spacer 190 is then deposited and etched toform sidewalls 191 of the gate stacks 171. An insulating material 195 isdeposited and planarized to fill-in the space between each gate stack171, as shown in FIG. 7. The insulating material 195 is typically formedof a boron-phosphate-silicon-glass (BPSG), but can be formed using anyother known insulating material. Subsequently, as shown in FIG. 8,conductive plugs 185 (also referred to as local interconnects) areformed. The conductive plugs 185 are formed by etching trenches ininsulating layer 195 and then filling the trenches with a metal orconductive material followed by planarizing any excess metal materialfrom above insulating material 195. Chemical mechanical planarization(CMP) or other known methods can be used to remove the excess metalmaterial from the surface of the insulating material 195. The conductiveplugs 185 may be any generally known conductive material that is used inthe art such as tungsten, platinum, gold, copper, aluminum, etc.

As shown in FIG. 9, an insulating material 174 is blanket depositedabove insulating material 195. Within insulating material 174, a trenchis etched and filled with a bit line 173 above one of the conductiveplugs 185. Then, another layer of insulating material 175 is depositedabove insulating material 195, the previously deposited insulatingmaterial 174 and the metal bit line 173. After insulating material 175is deposited, another set of trenches are etched into insulatingmaterial 175 and another set of conductive plugs 165 are formed. Theconductive plugs 165 are formed by filling the trenches with a metal orconductive material and planarizing the excess material from aboveinsulating material 175. Conductive plugs 165 extend conductive plugs185 to the top surface of insulating material 175.

Referring to FIG. 10, another insulating material 155 is formed aboveinsulating material 175 and conductive plugs 165. Then, trenches 147 areetched into insulating material 155 being a patterned area 155′ oneither side of the trenches. Any etching method known in the art can beused to etch the trenches. A metal material 145, preferably TiN, isdeposited within the trenches 147 and above insulating materials 155 and175. Then, a nitride material 135 is blanket deposited above metalmaterial 145. The nitride material 135 and metal material 145 are etchedback and removed, preferably a spacer etch, from above insulatingmaterial 155, as shown in FIG. 11.

The surface of insulating material patterned area 155′ and portions ofinsulating material 175 are exposed. FIG. 12 shows another insulatingmaterial 125 which is formed above insulating materials 155 and 175.FIGS. 13A and 13B illustrate a memory array 101 from a top-downperspective view at the processing state illustrated in FIG. 12. Asshown in the top-down perspective of FIG. 13A, the top edges of metalmaterial 145 remains exposed at the surface of insulating material 125.Materials 125, 145 are patterned etched perpendicular to the formedmetal material 145 lines and etched down to insulating area 155′ andportions of insulating material 175. Then, an insulating material isblanket deposited and is planarized to the top level of insulating area155′ (FIG. 13B) to expose the top edges of the formed metal material145. The resulting metal material 145, which forms a bottom electrode,is preferably formed in an I-shaped configuration (FIG. 2). Metalmaterial 145, however, can be formed in a number of various shapes giventhat the top surface or exposed surface of the shape provides a smallcontact area. Each contact, bottom electrode 145 is individuallyisolated by the insulating material 125. In other words, no directconductive path exists from one bottom electrode 145 to another.

The metal material 145, which serves as a bottom electrode to the formedphase change memory element 100 (FIG. 2), is visible at predeterminedspaced distances throughout the array 101. The remaining visible surfaceof the array 101 is comprised of insulating material 125 and areas 155′.

Referring back to FIG. 2, a phase change material 121 is thereafterdeposited onto the planarized bottom electrodes 145 and the insulatingmaterials 125, 155′. The phase change material 121 may be any of thevarious variable resistance materials listed above or may be a specificcomposition of GST. A top electrode 117 is deposited on top of the phasechange material 121 by any method. For example, the phase changematerial 121 and top electrode 117 may be blanket deposited one afterthe other and then etched back to the top level of insulating material155 to form memory elements with a top electrode. Thereafter, anotherinsulating layer 116 is formed and planarized and trenches 119, 113etched and filled with a conductive material 118. The top electrode 117is generally, but not required to be, formed of the same material as thebottom electrode 145. Metal material 118 connects and provides anelectrical connection through trench 119 to top electrode 117. Metalmaterial 118 is blanket deposited and patterned such that a portion isremoved from insulating material 116.

Referring to FIGS. 2 and 14, the final structure comprises electrodes A,B respectively forming a bit line and cell select line, phase changememory elements GST1, GST2, 122, 123 as well as transistor gates C, Dformed as respective word lines. FIG. 14 reflects an electricalschematic corresponding to the structure depicted in FIG. 2. Conductiveplugs 165, 185 form an electrical communication between bit line 173 andelectrodes 145, 117. Electrodes 145, 177 can continue electricalcommunication via conductive materials 118, 119 to periphery circuitrythrough periphery conductive plugs such as conductive plug 113. Thedescribed electrical communication paths correspond to the electricalschematics and illustrations of FIGS. 14-16.

The resulting phase change memory element has a low power consumptionand provides reduced current requirements by achieving a small bottomcontact area for electrodes 145 which, as noted, has an area of lessthan 1000 nm². Although having a smaller bottom contact area than otherphase change memory elements, the memory element still achieves anincreased current density that will heat the phase change material pastits melting point and quench it in an amorphous state. This maximizesthe current density at the bottom electrode interface to the phasechange material.

FIG. 14 also labels one memory cell 1315 as having a bit line 1340, acell select line 1320, a word line 1330 and a memory element 1310. Thememory element 1310 is connected to a cell select line 1320 via topelectrode B. The other memory element electrode 145 is connected to anaccess transistor 1350. The access transistor 1350 is gated by a wordline 1330. Bit line 1340 provides a source to the access transistor 1350and is connected to the memory element 1310 when the access transistor1350 is activated by the word line 1330.

The memory cell 1315 of FIG. 14 may be arranged in an array of likememory cell structures, as illustrated in FIG. 15. In FIG. 15, a memorydevice 1400 includes one or more arrays of memory bit structures 1315a-1315 p. The memory cell structures 1315 a-1315 p are arranged in rowsand columns. The rows and columns may be partially staggered or may bealigned as a simple parallel grid as in FIG. 15. The memory cellstructures 1315 a-1315 p along any given cell select line 1420 a-1420 ddo not share a common word line 1430 a-1430 d. Additionally, the memorycell structures 1315 a-1315 p along any given cell select line 1420a-1420 d do not share a common bit line 1440 a-1440 d. In this manner,each memory bit structure is uniquely identified by the combinedselection of the word line to which the gate of the memory cell accessdevice is connected, and the cell select line to which the memory cellis connected.

Each word line 1430 a-1430 d is connected to a word line driver in theform of a row decoder 1460 for selecting the respective word line for anaccess operation. Similarly, each cell select line 1420 a-1420 d iscoupled to a driver in the form of a column decoder 1450. The currentpassing through a selected memory bit structure 1315 a-1315 p ismeasured by sense amplifiers 1470 a, 1470 d connected respectively tothe cell select lines 1420 a-1420 d.

For simplicity, FIG. 15 illustrates a memory array having only four rowsof memory cell structures 1315 a-1315 p on four cell select lines 1420a-1420 d and four columns of memory cell structures 1315 a-1315 p onfour word lines 1430 a-1430 d. However, it should be understood that inpractical applications, the memory device 1400 has significantly morememory cell structures in one or more arrays.

It should be appreciated that the improved phase change memory cell 100may be fabricated as part of an integrated circuit. The correspondingintegrated circuits may be utilized in a typical processor system. Forexample, FIG. 16 illustrates a simplified processor system 1500 whichincludes a memory device 1400 employing improved phase change memorycells in accordance with the above described embodiments. A processorsystem, such as a computer system, generally comprises a centralprocessing unit (CPU) 1510, such as a microprocessor, a digital signalprocessor, or other programmable digital logic devices, whichcommunicates with an input/output (I/O) device 1520 over a bus 1590. Thememory device 1400 communicates with the CPU 1510 over bus 1590,typically through a memory controller.

In the case of a computer system, the processor system may includeperipheral devices such as removable media devices 1550 whichcommunicate with CPU 1510 over the bus 1590. Memory device 1400 ispreferably constructed as an integrated circuit, which includes one ormore phase change memory devices. If desired, the memory device 1400 maybe combined with the processor, for example CPU 1510, as a singleintegrated circuit.

It should also be appreciated that various embodiments have beendescribed as using a phase change material as an exemplary variableresistance material. The invention may also be used in other types ofresistive memory to improve current flow through whatever variableresistance material is used.

The above description and drawings should only be consideredillustrative of example embodiments that achieve the features andadvantages described herein. Modification and substitutions to specificprocess conditions and structures can be made. Accordingly, theinvention is not to be considered as being limited by the foregoingdescription and drawings, but is only limited by the scope of theappended claims.

What is claimed as new and desired to be protected by Letters Patent ofthe United States is:
 1. A variable resistance memory cell comprising:an insulating material element; a trench formed within the insulatingmaterial element; a first electrode within the trench formed within theinsulating material element, the first electrode having a top surface,and wherein an entirety of the top surface of the first electrodecomprises a contact area thereof and is less than 1000 nm²; aninsulating spacer material on a side of the electrode; a variableresistance material having a first surface and a second surface, thefirst surface abutting the top surface of the first electrode andportions of the insulating material element; and a second electrode inelectrical contact with the second surface of the variable resistancematerial.
 2. The variable resistance memory cell of claim 1, furthercomprising a memory bit line, at least one access transistor coupledbetween the bit line and one of the first and second electrodes, and acell select line coupled to the either of the first and secondelectrodes.
 3. The variable resistance memory cell of claim 1, furthercomprising a memory bit line, at least one access transistor coupledbetween the bit line and the first electrode, and a cell select linecoupled to the second electrode.
 4. The variable resistance memory cellof claim 1, wherein the variable resistance material is a phase changematerial.
 5. The variable resistance memory cell of claim 1, wherein thefirst electrode is L-shaped.
 6. The variable resistance memory cell ofclaim 1, further comprising a second memory cell formed adjacent to thefirst memory cell, the first and second memory cells being paired cellsin parallel electrical connection, each being contacted by separate wordlines.
 7. The variable resistance memory cell of claim 6, wherein thefirst and second memory cells share the same variable resistancematerial.
 8. A phase change memory element comprising: an insulatingstructure; a trench formed within the insulating structure; a bottomelectrode having a contact area of less than 1000 nm² and an L-shape andbeing formed within the trench formed within the insulating structure,an entirety of the top surface of the bottom electrode comprising thecontact area thereof; a variable resistance material with a firstsurface and a second surface, the first surface abutting the top surfaceof the bottom electrode and a top surface of the insulating structureproximate the bottom electrode; and a top electrode in electricalcommunication with the second surface of the variable resistancematerial.